1. Field of the Invention
The present invention relates to a method of producing a polycrystalline silicon rod, which is obtained by depositing silicon onto a carrier rod and cutting to size a molding produced in the process.
2. The Prior Art
A silicon rod produced in this way is suitable in particular as a feed rod in the production of a monocrystal by zone pulling (FZ-Process) or as a refilling rod for crucible pulling (CZ-Process). However, a precondition for this is that the silicon rod has at least one rod end which is free of cracks and flaking.
In order to produce the molding, thin carrier rods of silicon, so-called thin rods, are joined in a deposition reactor and heated electrically to reaction temperature by the direct passage of current. During the deposition of silicon, halogen-containing silicon compounds or silane decomposes from the gas phase at the surface of the thin rods. At the same time, the diameter of the thin rods increases. After a desired diameter has been reached, the deposition is ended and the molding produced is cooled down to ambient temperature and removed from the reactor. The molding is generally U-shaped, having two polycrystalline rods as legs and a bridge of polycrystalline silicon joining the legs. At their ends, the legs are grown with the electrodes for the current supply.
The temperature differences in the cooling molding are greater, the greater the diameter of the rods. The maximum temperature prevails in the cross-sectional center of the rods, the lowest temperature in the edge region of the rods. Because of this temperature profile, tensile and compressive stresses are produced in the rods during the cooling of the molding. Planes of equal stress lie cylindrically around the longitudinal axes of the rods.
In a conventional sawing process, the rods are fixed and cut through with a saw with vertical feed perpendicular to the stress planes and, in so doing, are separated from the electrodes and the bridge. Here, in particular in the case of rods with a large diameter, there is a considerable risk that the stress present in the rods will be released by the formation of cracks and flaking. If cracks and flaking occurs, the saw cut has to be repeated at a different rod position, to be specific until a cut face is obtained which is free of cracks and flaking. With each saw cut, the remaining rod becomes shorter and the yield becomes lower. The cracked pieces of rod which arise during the sawing have a lower quality, since they cannot be used as feed rods for zone pulling or as refilling rods for crucible pulling.
It is an object of the present invention to provide a method with which a polycrystalline silicon rod with at least one rod end that is free of cracks and flaking can be obtained with a high yield and cost-effectively.
The above object of the invention is achieved by providing a method of producing a polycrystalline silicon rod, which is obtained by depositing silicon on a carrier rod and cutting to size a molding produced in the process, which comprises rotating the polycrystalline silicon rod about its longitudinal axis and cutting through with a parting tool at least one point, and producing at least one rod end which is free of cracks and flaking. The rotating rod can be severed at one position or at a number of positions at the same time.
As a result of the rotation of the silicon rod, the action of the parting tool is carried out in such a way that the stress planes are cut through one after another. Thus, stress dissipation by crack formation is therefore avoided.
The method of the invention provides for the polycrystalline silicon rod to be cut initially from a molding which is grown by gas-phase deposition of silicon onto carrier rods in a deposition reactor. In this case, the electrodes and the bridge are preferably severed from the fixed molding by using a saw with a vertical feed. The mechanical processing according to the invention of the silicon rod is then carried out, as a result of which the silicon rod is given, at the severed position, a rod end which is free of cracks and flaking.